It is known to make rubber-modified styrene/maleic anhydride copolymers by dissolving a rubbery additive in the monomer mixture and copolymerizing. Generally, the rubbery additives are soluble in the vinyl aromatic monomer or in the mixture of monomers. However, in the past the addition of as much as about 16 or 17% by weight of the total copolymer of rubber is all that can be tolerated in a commercial-type reactor due to the high viscosity of the monomers when admixed with a rubbery additive.
The blending of rubbery additives to a styrene/maleic anhydride copolymer or to a rubbery modified copolymer is also known. U.S. Patents such as Nos. 4,097,550, 4,097,551, 3,641,212 and 2,914,505 all relate to the improvement of thermoplastic compositions by rubbery additive admixtures but all of these patents teach the blending of the rubbery additives with the copolymer.
U.S. Pat. No. 3,919,354, issued Nov. 11, 1975, shows the making of a rubber-modified styrene/maleic anhydride copolymer having impact resistance and that is prepared by providing a solution of the rubber in styrene, initiating free radical polymerization and then adding maleic anhydride at a rate substantially less than the rate of polymerization of the styrene. However, this patent does not teach or achieve the greater product or improvement obtained by the present invention, nor the improvements in the process of making such improved product. Further, the patent states that the non-equimolar random copolymers are obtainable, preferably by the steps described in the patent as set forth above. In fact, the patent states that a polymer prepared from 75 mole percent styrene and 25 mole percent maleic anhydride by admixing the monomers with suitable diluent and catalyst, heating until polymerization is about complete, will yield a polymer or polymer mixture which is not suitable for the practice of the invention.
It has been discovered by the present invention that when two different rubbery additives are used and are present during the polymerization of the copolymer that a striking phenomenon occurs. As the copolymerization occurs, a lower viscosity of the complete mixture is evident, the process may thus be more readily carried out with a marked decrease in the energy requirements. Secondly, an improved product is made in that not only is heat resistance improved, but, in addition, impact is significantly improved and the product is upgraded to the quality of the so-called engineering materials which are considered specialty products with a correspondingly high price.
The surprising discovery of using at least two rubbery additives and having the same present during the polymerization of the copolymer permits the permissible amount of rubber additives to be greatly increased and this in turn correspondingly improves the impact properties of the resulting product. As an additional feature, it has been found that the processability of the resultant product, as well as its glossy surface appearance, is greatly improved.